1. Field of the Invention
The present invention relates to a phosphorized copper anode used for electroplating, by the use of which particles are not attached to or generated on a plating surface of a cathode. More specifically, the present invention relates to a phosphorized copper anode used for electroplating in order to form a copper wiring for a semiconductor device.
2. Description of Related Art
In general, it is known that a phosphorized copper anode may be used as an anode for electroplating copper. As an example of a phosphorized copper anode for electroplating, one which includes 350-700 ppm of phosphorus and 2-5 ppm of oxygen, besides copper and inevitable impurities, is known as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 8-67932.
This conventional phosphorized copper anode for electroplating is used for plating copper on a drum for gravure printing. The copper anode is manufactured by: preparing electrolytic copper having a purity of more than 99.99%; dissolving the electrolytic copper in a shaft kiln under a CO+N2 atmosphere; supplying the resultant molten metal to a holding furnace; adding phosphorus to the molten metal in the holding furnace; immediately casting the molten metal to form an ingot of a predetermined size; removing a top portion of the ingot and subjecting the ingot to a forging process; and cutting the ingot to a predetermined size after carrying out a facing process thereon. The phosphorized copper anode for electroplating produced in this manner has a processed structure.
On the other hand, aluminum alloys have been used as a wiring material for semiconductor devices for a long time.
These days, however, from the viewpoint of decreasing the size of semiconductor devices and increasing the density thereof, copper, whose resistivity is lower than that of aluminum by almost 40%, is used for a wiring material for semiconductor devices instead of aluminum alloys. However, if wiring is formed on a semiconductor device by means of a copper plating method using a conventional phosphorized copper anode, a black film-like substance is formed on the surface of the copper anode, becomes separated from the anode surface during the electroplating process, and is left floating in the plating bath. It is now understood that a part of the black film-like substance attaches onto a copper thin film, which is formed on the silicon wafer surface of the cathode side and becomes copper wiring by the electroplating process, as a particle and causes problems.
The inventors of the present invention, in consideration of the above problems, carried out diligent studies to obtain a phosphorized copper anode for electroplating, by the use of which particles do not become attached to the surface of a copper thin membrane when wiring is formed on a semiconductor device by means of copper plating.
As a result, the inventors of the present invention found that, as compared with a conventional anode, if a phosphorized copper anode for electroplating is prepared while decreasing the oxygen content thereof to a level between 0.1 ppm or greater and less than 2 ppm, making the structure thereof a fine recrystallized structure, and adjusting the average grain size thereof after recrystallization to fall in the range between about 10-50 xcexcm, then there is very little separation of the black film-like substance from the surface of the copper anode on which it is formed during the electroplating process when the resultant copper anode is used for forming copper wiring by means of electroplating. Accordingly, it was also found that when copper wiring is formed on a semiconductor device by using the phosphorized copper anode for electroplating, there is almost no attachment or generation of particles on the copper wiring.
The present invention was completed based on the above findings and has an object to provide a phosphorized copper anode for electroplating comprising: 20-800 ppm of phosphorus, between 0.1 and less than 2 ppm of oxygen, and the balance being high purity copper of 99.9999% by mass or higher, wherein the average grain size of the anode after recrystallization is in the range between about 10 and 50 lm.
If the amount of phosphorus contained in the copper anode for electroplating is less than 20 ppm, copper particles may be generated during the electroplating process, which is not preferred. In such contrast, if the content of phosphorus is greater than 800 ppm, it is also not preferable since its electric conductivity is decreased, and the electric energy loss is increased. Thus, the content of phosphorus in the phosphorized copper anode for electroplating according to the present invention is determined to be between 20 and 800 ppm. Moreover, the amount of phosphorus contained in the copper anode for electroplating according to the present invention is preferably in the range between about 250 and 550 ppm.
On the other hand, although it is preferable that the amount of oxygen contained in the phosphorized copper anode for electroplating be as low as possible according to the present invention, it is economically inefficient to decrease the amount of oxygen to be less than 0.1 ppm. On the other hand, if the amount of oxygen is 2 ppm or greater, it is not preferable because the black film-like substance formed on the surface of the phosphorized copper anode tends to be easily separated. Thus, the amount of oxygen contained in the phosphorized copper anode for electroplating is determined to be between 0.1 ppm or greater and less than 2 ppm. The amount of oxygen contained in the phosphorized copper anode for electroplating is preferably in the range between about 0.4 and 1.2 ppm.
The structure and the grain size of the phosphorized copper anode for electroplating greatly affect the separation of the black film formed during an electroplating process. It is preferable that the structure of the phosphorized copper anode for electroplating according to an embodiment of the present invention be a recrystallized structure. The smaller the grain size thereof, the more preferable it is. However, it costs too much to make the average grain size after recrystallization less than 10 xcexcm, and hence, it is economically not preferred. On the other hand, if the average grain size after recrystallization exceeds 50 xcexcm, the black film-like substance formed on the surface of the phosphorized copper anode tends to become separated, which is not preferable. Accordingly, the average grain size after recrystallization of the phosphorized copper anode for electroplating according to an embodiment of the present invention is determined to be in the range between about 10 and 50 xcexcm. It is preferable that the average grain size after recrystallization of the phosphorized copper anode for electroplating be in the range between about 15 and 35 xcexcm.
It is preferable that the phosphorized copper anode for electroplating according to an embodiment of the present invention be constructed using electrolytic copper having a purity of more than 99.9999%. This is because if the copper anode is constructed by using electrolytic copper having a purity of more than 99.9999%, the tendency for the black film to become separated therefrom is significantly reduced as compared with the case where a copper anode is constructed using electrolytic copper having a purity of more than 99.99%.
The phosphorized copper anode for electroplating according to an embodiment of the present invention may be constructed by: preparing electrolytic copper having a purity of more than 99.9999%; placing the electrolytic copper into a carbon crucible; dissolving the electrolytic copper under an inert or reduced gas atmosphere having a dew point of xe2x88x9210xc2x0 C. or lower; adding phosphorus to the resultant molten metal and casting the molten metal at a temperature between 1150 and 1300xc2x0 C. to form an ingot of a predetermined size; removing a top portion of the ingot and applying heat; subjecting the ingot to a forging process and carrying out a cold rolling process to 20-80% draft; applying heat in the range between about 300 and 500xc2x0 C. for about 20 minutes to 4 hours so as to adjust the average grain size after crystallization to be in the range between about 10 and 50 xcexcm; and cutting to a predetermined size after carrying out a facing process.